A. Field of the Invention
The present invention relates to a process for converting oxygenate, such as methanol and/or dimethylether, to olefins in a reactor over a molecular sieve catalyst such as ZSM-34 and SAPO-34 wherein oxygenate is introduced to a catalyst bed at multiple injection points along the flow axis of the reactor. The process is especially useful for increasing ethylene selectivity.
B. Description of the Prior Art
Olefin manufacturing processes, such as steam cracking, generally operate at low pressure, high temperature, and with diluents, such as steam, to enhance yields of light olefins which are thermodynamically favored by these conditions. In producing light olefins, the selectivity to ethylene can be increased to some extent by increasing reactor severity, e.g. by operation at low pressure, high temperature, and/or by diluent addition. However, as reactor severity is increased total olefin production is decreased. Moreover, the production of paraffins, i.e., methane, ethane, propane, etc., aromatics and other less desirable components will also increase. Similarly, diluent addition entails significant expense. For example, with steam diluent, the expense of generating the steam and the costly equipment to condense the steam for product recovery must be considered with the additional revenues attained for the higher olefin yields. Furthermore, all equipment must be increased in size to handle the processing of diluent as well as reactive feed. With methanol conversion processes, these expenses generally negate using diluents in significant quantity.
In an effort to improve yields in various reaction procedures, stagewise injection of reagents has been used in various fixed bed processes. For example, U.S. Pat. Nos. 4,377,718 and 4,761,513 describe toluene alkylation processes wherein the alkylating reagent is fed at different stages between fixed beds. Likewise, U.S. Pat. No. 3,751,504 discloses a similar procedure, using multiple injection ports, for preparing ethylbenzene using a fixed bed catalyst reactor. U.S. Pat. No. 5,120,890 discloses multiple reactant injection locations into separate fixed beds in a process for reducing benzene and toluene content in light gasoline streams. U.S. Pat. Nos. 3,751,504; 4,377,718; 4,761,513; and 5,120,890 are each entirely incorporated herein by reference. In these fixed bed processes, one can easily separate the catalyst load into several different and discrete zones. During use, product from one zone is mixed with additional methanol, and this mixture is fed to the subsequent zone. One way of providing these separate and discrete zones includes placing each zone in a separate reactor vessel, wherein additional reagent(s) is (are) injected between adjacent zones. This procedure suffers from the drawback that considerable expense is involved in providing separate reactor vessels and the associated hardware for running this type of system. Additionally, fixed bed reactors are disadvantageous for exothermic reactions because of the potential negative impact of exotherms on product selectivity. Reactor stability concerns with fixed beds also require that the temperature rise per catalyst bed be limited. This could necessitate a large number of beds to accommodate the heat of reaction.
U.S. patent application Ser. No. 09/166,188, filed Oct. 5, 1998 describes multiple methanol injection into a fluid bed to increase p-xylene selectivity and methanol utilization.